1. Field of the Invention
This invention relates to the use of real-time holographic (RTH) compensated imaging, and more particularly to compensated imaging using a double-pumped phase-conjugate mirror (DPCM).
2. Description of the Related Art
The DPCM is a new type of phase conjugate mirror (PCM) that has been developed over the past few years. Unlike prior PCM systems, in which the input beams had to be mutually coherent, the DPCM is capable of coupling mutually incoherent beams in a bidirectional, dynamic holographic link. The basic operation of a DPCM is illustrated in FIG. 1, in which a pair of mutually non-coherent input beams 2 and 4, which may be generated by respective lasers, are applied at different angles at a two-beam photorefractive coupling medium 6, typically barium titanate (BaTiO.sub.3). The two beams establish an optical grating within the crystal 6, with the input beam 2 emerging as an output beam 8 in a direction opposite to beam 4; input beam 4 emerges as an output beam 10 in a direction opposite to beam 2. Output beam 8 has the spatial profile of the phase conjugate of beam 4, but retains the spectral properties of input beam 2. Similarly, output beam 10 assumes the spatial profile of the phase conjugate of input beam 2, but retains the spectral properties of input beam 4. When input beams 2 and 4 are of the same wavelength, the output beams 8 and 10 are parallel to the input beams. In a variation referred to as a double color-pumped PCM, the input beams 2 and 4 have different wavelengths, and the output beams 8 and 10 are offset respectively from beams 4 and 2 by equal non-zero angles.
A general discussion of DPCMs and other photorefractive oscillators is provided in Fischer et al., "Photorefractive Oscillators", IEEE Journal of Quantum Electronics, Vol. 25, No. 3, March 1989, pages 550-569. A specific application of the DPCM to heterodyne detection is discussed. This is a temporal detection scheme that uses a DPCM, with two input beams, to generate a local oscillator beam whose wavefront is matched to an incoming beam that is temporally modulated. An experimental demonstration of Fischer et al.'s DPCM heterodyne detection system, with an added self-pumped conjugator for improved wavefront matching and auto-aligning, is described in Adams et al., "Wide-field-of-view heterodyne receiver using a photorefractive double phase-conjugate mirror", Optics Letters, Vol. 16, No. 11, Jun. 1, 1991, pages 832-834.
A DPCM has also been proposed for beam cleanup in a pulsed laser system, Chang, "Spatial-mode cleanup of a pulsed laser beam with mutually pumped phase conjugation with a cw reference", Optics Letters, Vol. 15, No. 23, Dec. 1, 1990, pages 1342-1344. A DPCM is used to provide shared gratings with a distorted pulsed laser beam and an undistorted reference cw beam. Only two beams are used: the distorted pulsed beam and the "clean" reference cw beam. There is no spatial or temporal modulation, or other encoding, of the pulsed beam.
Systems have also been proposed for various achromatic beam coupling systems in photorefractive crystals. The use of gratings in photorefractive crystals for an achromatic operation is proposed and modeled in Cronin-Golomb, "Achromatic volume holography using dispersive compensation for grating tilt", Optics Letters, Vol. 14, No. 23, Dec. 1, 1989, pages 1297-1299. White-light two-beam coupling in photorefractive crystals is demonstrated, using Cronin-Golomb's proposal, in Rabinovich et al., "Photorefractive two-beam coupling with white light", Optics Letters, Vol. 16, No. 10, May 15, 1991, pages 798-800. The use of two-beam coupling with achromatic gratings is discussed, and its effect on different read and write laser wavelengths is evaluated for barium titanate, in Kong et al., "Experimental study of achromatic volume holography with dispersive compensation in barium titanate", Optics Letters, Vol. 17, No. 4, Feb. 15, 1992, pages 297-299. In each of these publications only two input beams are considered, and no spatial or temporal modulation or other encoding of the inputs is discussed.
Another proposed application is for DPCM optical interconnects, with a beam fan-in and fan-out capability using a strontium barium niobate crystal, Cronin-Golomb, "Dynamically programmable self-aligning optical interconnect with fan-out and fan-in using self-pumped phase conjugation", Applied Physics Letters, Vol. 54, No. 22, May 29, 1989, pages 2189-2191. The ability to transfer modulated information from a number of input ports to various combinations of output ports is described. Multiple local beam sources and multiple remote beam sources are used to write gratings in the crystal. The remote beams are assumed to be out-of-phase with each other, and the local beams are also assumed to be out-of-phase with each other. The proposed system segregates each input beam to a corresponding single or multiple set of output beams for applications such as neural networks, and image transmission through thick distortion media. For the image transmission application, the system sequences through a training cycle on a pixel-by-pixel basis. One concern of this system is that, unless the gratings for the various pixels are properly registered relative to each other, the overlap of gratings from different pixels causes artificial phase shifts between successive pixels. The proposed solution, which is to train two beam pixels at a time and to overlap successive training pairs, has the drawback of reducing the output resolution. Furthermore, because it is restricted to processing only one pixel at a time, or at best overlapping sequential pairs of pixels, this approach is not applicable to real-time processing of multiple-pixel images.
An approach similar to that described in the Cronin-Golomb Applied Physics Letters article is disclosed in Schamschula et al., "Adaptive optical interconnection", Optics Letters, Vol. 16, No. 18, Sep. 15, 1991, pages 1421-1423. A DPCM is used to compensate for thermal, vibrational and other environmental aberrations by establishing bidirectional beam fan-in, fan-out optical interconnections. This on-axis system uses beam transmission holograms, along with a dual-wavelength operation, for a simple and high throughput system. As with Cronin-Golomb, the input beams are basically treated as single-pixel beams, without multi-pixel modulation. The use of a DPCM to couple two different infrared lasers together in a InP:Fe crystal has also been reported, in Vieux, "Double-phase conjugated mirror and double color pumped oscillator in photorefractive InP:Fe", Applied Physics Letters, Vol. 58, No. 25, Jun. 24, 1991, pages 2880-2882.
Despite the work that has been done with DPCMs, there have been no proposals for a real-time adaptive optics system for removing aberrations picked up during the transmission of an image bearing beam. Adaptive optics systems capable of correcting for phase aberrations acquired during the transmission of an image-bearing beam are known, but such systems require that at least two of the beams used be mutually coherent. In practice, this is quite difficult to achieve, and imposes significant design restrictions. Moreover only phase aberrations have been compensated; in general, path distortions can result in both phase as well as amplitude distortion upon transmission. In one approach, counter-propagating pump beams are applied to a four-wave mixing medium, and a probe beam is also directed onto the mixer after being transmitted through an aberrating medium. An image is placed on one of the pump beams and transferred in the mixing medium to the probe beam, so that the conjugate of the probe beam transmitted back through the aberrating medium bears the image and has the aberrations removed during the reverse transit. Four-wave mixing in general is described in Saleh et al., Fundamentals of Photonics, John Wiley & Sons, 1991, pages 7561.varies.761. The system will only work, however, if the image-bearing pump beam is coherent with the probe beam.
Another prior adaptive optics technique is to transmit a reference beam and an image-bearing beam through an aberrating medium, and then use the two aberrated beams to write a hologram. The aberrations of the two beams in effect cancel, and a third beam is used to read out the undistorted image from the hologram. This technique requires that the original reference and image-bearing beams be mutually coherent.